cmd/link/internal/mips64: fix use of -s flags

My flags change reversed the meaning of -s within mips64's
linker code. This should fix that.

Change-Id: Ia24002469e557fb29badfd830134e61c1dd7e16e
Reviewed-on: https://go-review.googlesource.com/27555Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/link/internal/mips64/asm.go

@@ -226,7 +226,7 @@ func asmb(ctxt *ld.Link) {
 
 	ld.Lcsize = 0
 	symo := uint32(0)
-	if *ld.FlagS {
+	if !*ld.FlagS {
 		// TODO: rationalize
 		if ctxt.Debugvlog != 0 {
 			fmt.Fprintf(ctxt.Bso, "%5.2f sym\n", obj.Cputime())